Just think how much more they could have sunk had they had a good long range reliable torpedo?
US Sub launched torpedoes were solved and improved at the end of 1943 and early 1944. In 1944, the notable advance in US torpedo technology was the introduction of a wakelesss electric torpedo. Not as fast as a conventionally powereed torpedo, but untrackable, especially at night.
The other big problem for the USN was the faulty exploder they were using. It took over 20 months to work this problem out.
There were other problems. US tactical doctrine was faulty, subs tended to approach from the wrong angles which meant many targets simply walked away from the slow moving, short ranged "S" class subs. Many of the prewar skippers were repoprtedly too timid in their attacks, breaking off well before they should.
Lastly the higher levels of leadership were apparently fairly ordinary in their strategic management of their submarine flotillas., poor placement, a fialure to identify problems of equipment and training being among them
Japanese ASW training was woeful. It did show some signs of improvement from 1943 onward. In 1941 it was very poor however. On the other hand, Japanese shipping losses were very minor until August 1942. Ther Japanese captured more shipping then they lost in the initial campaigns.
US subs based in the Phillipines suffered an early, fatal blow in the campaign when the supply of torpedoes was destroyed very early on with a bomb hit at the Cavite Navy Yard.
After the fall of the far east, the majority of US subs lacked the range and endurance to be effective. It required the large scale introduction of the longer ranged Gato class subs to solve this problem.
Once the problems were solved US employment of submarines was nearly faultless. The USN had a clear understanding of where their subs could be best utilised (ie sinking the Japanese merchant fleet), and went about it with calculated efficiency